Scientists have found a way to track minutely-differing strains of the 'superbug' MRSA as they spread, a finding that could aid efforts to control the deadly bacteria.

An international team lead by researchers from Britain's Wellcome Trust Sanger Institute used very high-throughput gene sequencing machines to compare individual MRSA bugs from patients and show precisely how they were genetically related.

Methicillin-resistant Staphylococcus aureus (MRSA) causes infections such as blood poisoning and pneumonia and can result in death. It is one of a group of drug resistant bacteria, or superbugs that are a major problem in hospitals around the world.

Dr Stephen Bentley, who led the study published by the journal Science, says the new technology has allowed scientists for the first time to find precise differences in strains of the bug - a "fundamentally important" step to tackling infection.

"It allows researchers and public health officials to see how infections are spread, from person to person, from hospital to hospital, from country to country," he says.

Until now, even the best methods for identifying genetic differences between bacteria have been unable to pick up tiny differences - leaving uncertainty about how infections spread.

The success of the new method relies on comparing whole genetic codes, the scientists say. The ability to track strains in this way will help researchers understand how strains can spread so rapidly, and should lead to new control strategies, not only for MRSA but also for other emerging superbugs.

Evolutionary tree

The researchers looked at 62 MRSA samples. One set of 42 was taken from hospitals in North and South America, Europe, Australia and Asia from patients who became infected with MRSA between 1982 and 2003, and 20 were from a hospital in Thailand, from patients who developed MRSA within seven months of each other.

"We wanted to test whether our method could successfully zoom in and out to allow us to track infection on a global scale - from continent-to-continent, and also on the smallest scale - from person-to-person," says Simon Harris of the Sanger Institute.

The team sequenced the whole genomes of all the samples and were able to spot single-letter changes in the genetic code and identify differences between even the most closely related bugs.

From the results they created an "evolutionary tree" which showed that MRSA infections are often clustered in locations, but can be spread across borders by patients travelling between place and another and visiting different hospitals.

Drug-resistant bacteria kill about 25,000 people a year in Europe and about 19,000 in the United States. The European Centre for Disease Prevention and Control says superbug infections cost 900 million euros (A$1.4 billion) a year in extra hospital time and 600 million euros a year (A$930 million) in lost productivity.

Dr Sharon Peacock of Cambridge University, who also worked on the study, says the work could "flag up hotspots for MRSA transmission ... and these could then be examined to improve infection control strategies."